2 20 García-Suárez F et al., ΦYTON 76 (2007) Resumen. En este trabajo contribuimos al conocimiento de la fenología reproductiva de Stenocereus stellatus (Pfeiffer,Riccobono) el cual es un cacto tipo columnar que produce frutos con una gran perspectiva comercial conocidos como pitayas. Este tipo de pitayas son producidos en huertos de traspatio en algunas regiones del trópico seco Mexicano y son comercializados localmente. Estos frutos solo son producidos en la parte apical de los brazos de la planta. Nuestros resultados muestran que el desarrollo de los frutos de pitaya está fuertemente asociado al período lluvioso del año. Este comportamiento es diferente de la estrategia reproductiva de otras especies de cactos columnares que producen otro tipo de pitayas y que crecen en la misma región ecológica. Palabras clave: Cactos columnares, fruto de pitaya, fenología, estrés hídrico. INTRODUCTION The understanding of the environmental factors that affect fruit production is very important for the optimal utilization of resources. One of the most important phenomenon in horticulture is the phenology of species and, in cacti, phenology is particularly poorly known. Rainfall amount and distribution can significantly affect the reproductive behavior of many plant species. Sexual reproduction of the majority of the species in the deciduous tropical forest (including some cacti) takes place during the middle or at the end of the dry season (Rzendowsky, 1978). Janzen (1978) establishes a strong correlation between the time of sexual reproduction of tropical plants and the existence and activities of their pollinators and dispersal agents (insects and animals). In arid ecosystems most plants respond to precipitation. The pulsereserve model addresses the response of individual plants to precipitation and predicts that there are biologically important rain events that stimulate plant growth and reproduction (Ogle & Reynolds, 2004). There is increasing commercial interest in fruits of some columnar cacti which have an economic value in national and international markets. This has motivated the establishment of commercial orchards; such is the case for fruits known regionally as pitayas. Pitaya is a generic name for fruits produced by different Mexican cacti species, such as, Stenocereus griseus (Haw) and Stenocereus stellatus (Pfeiffer) Riccobono in Oaxaca and Puebla (Piña-Lujan, 1977); Stenocereus queretaroensis in Querétaro, Jalisco and Michoacán (Pimienta-Barrios & Nobel, 1995; Mizrahi et al., 1997),

3 Pitaya fruit growth is associated to wet season in Mexican dry tropic. 21 and Stenocereus fricci in Michoacán (Rebollar-Alviter et al., 1997). Stenocereus stellatus (Pfeiffer) Riccobono is a columnar cactus endemic to central México. Its location is centered on the Tehuacán Valley and La Mixteca Baja, in the states of Puebla, Oacxaca and Guerrero (Casas et al., 1999). The fruits produced by these plants under natural conditions are important economic income sources to different marginal communities, and are a food source for human and animals. The cultivation of this crop is feasible using relatively low inputs (Pimienta-Barrios & Nobel, 1998). In this paper we report the phenological observations of pitaya plants (Stenocereus stellatus) in a rural orchard. We show that the pattern of fruit growth of this cactus is related to the rainy season, which contrasts with other pitaya species (Stenocereus queretaroensis, Stenocereus griseus) where part of the reproductive period occurs during the dry season. These pitaya species are also components of the Mexican Tropical Deciduous Forest. MATERIALS AND METHODS Study site. The study was conducted at a rural orchard in Chiautla, Puebla, México. The orchard was formed using different wild plant parts ( arms ) established by vegetative propagation (López-Gómez et al., 2000). Phenological observations. Eighty adult individuals plants were marked and phenological observations were taken monthly for one year, starting in February Fruit length and diameter were taken using a vernier every 15 days for the different fruits produced in the selected plants. Fruit volume was calculated according to the formula V= r 2 h where V is fruit volume, r is the fruit radius (r = diameter/2), and h is fruit length. We tagged and determined the date when a group of fruits (group 1; n = 250 fruit) first appeared in the reproductive period in different arms of each plant. We also tagged a second fruit group (group 2; n = 100 fruits) which formed 15 days after the first group. Statistical analysis. Regression and correlation analyses were made using the software SYSTAT version 9.01 to establish the relationship between rainfall during the wet season and the rate of fruit growth. The rate of fruit growth was determined as the change of the fruit volume. Rainfall data were obtained from a weather station maintained by the Comisión Nacional del Agua in Chiautla, Puebla, México.

4 22 García-Suárez F et al., ΦYTON 76 (2007) RESULTS AND DISCUSSION Phenology data were used to build a phenological diagram (Fig. 1a). Flowers and fruits in this cactus only appeared in the apical part of the arms. The flowering period of this species began in May and June and ended in August, while the fructification period went from June through November. The highest number of green (immature) fruits were observed in August, while the ripe fruits were found in August and September. Fruits ripened days after anthesis. This reproductive period coincided with the rainy period of the year. Figure 1b shows the average rainfall for a 10-year period in the Chiautla region. The reproductive behavior of this cactus is different from that of other cacti species that produce pitayas like Stenocereus queretaroensis and S. griseus, which bear fruits during the dry period of the year (Benito-Bautista et al., 1992; Pimienta-Barrios & Nobel 1998; Areta-González et al., 1999). We determined the relationship of fruit growth versus precipitation in the pitaya of S. stellatus using a biphasic model for the group 1 of fruits, and a simple lineal regression for the fruits of group 2. The parameters of the correlation are shown in Table 1 and plotted in Fig. 2. Table 1. Correlation parameters for the relationship of pitaya fruit growth with rainfall. Tabla 1. Parámetros de correlación para la relación del crecimiento del fruto de pitaya con la lluvia. Y represents fruit volume (cm 3 ), X expresses the number of days. The parameters a (a,a1,a2) and b (b,b1,b2) represent the ordinate and straight slopes. R is the correlation coefficient. Y representa el volumen del fruto (cm 3 ), X expresa el número de días. Los parámetros a (a, a1, a2) y b (b, b1, b2) representan la ordenada y las pendientes lineales. R es el coeficiente de correlación.

6 24 García-Suárez F et al., ΦYTON 76 (2007) Fig. 2. Patterns of pitaya fruit growth and rainfall during the summer [wet season (June-September)]. Clear circles represent the monthly cumulative rainfall. Black circles represent the fruit growth rates of group 1, while black squares represent the fruit growth rate of group 2. Fruits of group 1 reached a bigger size and better quality than those of group 2. Fig. 2. Modelos del crecimiento del fruto de pitaya y de la lluvia durante el verano (estación húmeda de Junio a Setiembre). Los círculos vacíos representan la lluvia mensual acumulativa. Los círculos negros representan la tasa de crecimiento del fruto del grupo 1, mientras que los cuadrados negros representan la tasa de crecimiento del fruto del grupo 2. Los frutos del grupo 1 alcanzaron un mayor tamaño y mejor calidad que aquellos del grupo 2. For the first group of fruits (group 1) there were two stages in fruit development based on the pattern of growth rate over time. In the first stage, fruit growth rate was 1.01 cm 3 /d and the timing of this stage coincided with the beginning of the rain period and the first maximum in rainfall. The second stage of fruit growth is probably a consequence of a rise in the rainfall amount (June). In the second stage, fruit growth rate was about four times (4.03 cm 3 /d) as high as in the first stage (Fig. 2). This fruit growth pattern is different from of a double sigmoid pattern reported for opuntia fruits (De la Barrera & Nobel, 2004). Fruits of group 2 showed a growth rate that was almost constant during fruit development (Table 1, Fig. 2). In addition, fruits from group 2 reached a sma-

7 Pitaya fruit growth is associated to wet season in Mexican dry tropic. 25 ller size, ripened faster and had a lower eating quality than those from group 1. Possibly this growth behavior is a consequence of a low fruit osmotic potential as a result of the accumulation of osmolytes that would create a driving force for increasing fruit water uptake immediately after a rainfall event. Several horticultural species (e.g., tomato, melon, cherries) show an increased water uptake by the fruit after irrigation or a rainfall event preceded by a period of water stress (Huang et al., 2000; Kozlowski et al., 2002; Moing et al., 2004 ). The reproductive behavior of plants after a precipitation event has been explained in the ecological context of the pulse reserve model, which addresses the response of some desert plants to precipitation. This model predicts that there are biologically important rainfall events that stimulate plant growth and reproduction. These pulses of precipitation may play a key role in long term plant function and survival in desert plants (Ogle & Reynolds, 2004). The different patterns of response to rainfall events by pitaya-producing cacti may have horticultural applications. Mixed orchards could be established combining species such as S. griseus or S. queretaroensis which produce fruits during the dry season, with species such as S. stellatus or S. fricii which produce fruits during the wet season. REFERENCES Arceta-González, I.E., E. Pimienta-Barrios, J. Zañudo-Hernández & V. Olalde-Portugal (1999). Caracterización fenológica y fisiológica de una población silvestre de Pitayo Stenocereus queretaroensis (Weber) Buxbaum asociada con micorrizas vesículo-arbusculares. Agrociencia 33: Benito-Bautista, P., R. López-Gómez & L. Chávez-Sánchez (1992). Biología y aprovechamiento de los frutos de tres cacatáceas: Stenocereus griseus, Stenocereus stellatus y Escontria chiotilla. Cuadernos de Investigación. CIIDIR-Oaxaca. Instituto Politécnico Nacional. México. 42 p. Casas, A., A. Valiente-Banuet, A. Rojas-Martinez & P. Dávila (1999). Reproductive biology of the process of domestication of the columnar cactus Stenocereus stellatus in Central Mexico. American Journal of Botany 86: De la Barrera, E. & S.P. Nobel (2004). Carbon and water relations for developing fruits of Opuntia ficus-indica (L.) Miller, incluiding effects of drought and gibberellic acid. Journal of Experimental Botany 55: Huang, X.M., H.B. Huang & F.F. Gao (2000). The growth potential generated in citrus fruit under water stress and its relevant mechanisms. Scientia Horticulturae 83: Jansen, D.H. (1978). Seedling patterns of tropical trees. In: Tomilson, P.B. and Zimmermann M.H. (eds) pp Tropical trees as living systems. Cambridge University Press. London, UK. 544 p. Kozlowski, T.T. & S.G. Pallardy (2002). Acclimation and adaptive responses of woody plants to environmental stresses. Botanical Review 68:

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